The principal objective of radiation spectroscopy is to record a spectrum from a radiation detector. The spectrum provides information about the incident radiation and the response of the radiation detector. A common approach to the securing and recording of differential pulse-height spectra is to use a multi-channel pulse-height analyzer, sometimes referred to as a “multi-channel analyzer (“MCA”), as fully described in the referenced textbook Radiation Detection and Measurement, Wiley, Third edition, 2000, by Glenn F. Knoll. The differential pulse-height spectrum is recorded as a differential number of pulses with pulse heights within a small amplitude interval. This interval is nearly constant over the full range of analyzed pulse heights and is called “channel width.” The full pulse-height range divided by the channel width determines the total number of channels of the pulse-height analyzer. Typically, the channels are consecutively numbered, using integer numbers starting from zero.
FIG. 1 depicts a typical radiation-spectroscopy system based on a multi-channel pulse-height analyzer. The radiation strikes a RADIATION DETECTOR (10) and deposits partial or full energy into the detector. (In referring to prior-art components and sub-assemblies, upper-case letters, and reference numerals in parentheses, are being employed to set the stage for the description of the present invention in later paragraphs.) The interaction between the radiation and the RADIATION DETECTOR is often referred to as a “detector event.”
The RADIATION DETECTOR responds to the deposited energy by one or more different mechanisms, such as generating electrical charge or a short current pulse, emitting short pulses of light, changing RADIATION DETECTOR temperature, or creating acoustic waves. In all cases for the purpose of pulse-height spectroscopy, use is made of short electrical pulses derived from the RADIATION DETECTOR signal.
The electrical pulses are processed by a SIGNAL PROCESSOR (20.) The SIGNAL PROCESSOR provides means to fully collect the signal from the detector, reduce the noise, suppress “pulse pile-up” and to accommodate other functions that enhance the quality of the recorded spectra. A processor may work in both analog and/or digital domains. In both cases, the pulse height is proportional to the magnitude of the signal from the detector.
A PULSE-HEIGHT ANALYZER (30) measures the pulse heights of the pulses from SIGNAL PROCESSOR (20). The main functions of PULSE-HEIGHT ANALYZER (30) are peak detection and analog-to-digital conversion of the pulse height. If a pulse height is within a given channel width, then the corresponding CHANNEL ADDRESS is generated at the output of PULSE-HEIGHT ANALYZER (30). The CHANNEL ADDRESS is an integer number obtained as a result of a single pulse-height measurement performed by PULSE-HEIGHT ANALYZER (30) and can be viewed as a digitized pulse height.
The CHANNEL ADDRESS is fed to a SPECTRUM MEMORY UNIT (40). SPECTRUM MEMORY UNIT (40) builds the spectrum histogram by incrementing the “channel content” of the channels indicated by the CHANNEL ADDRESSES from PULSE-HEIGHT ANALYZER (30). In some cases, SPECTRUM MEMORY UNIT (40) scales down the CHANNEL ADDRESS so that the spectrum resolution is less than the resolution of the PULSE-HEIGHT ANALYZER (30).
FIG. 2 shows a block diagram of SPECTRUM MEMORY UNIT (40). The CHANNEL ADDRESS is fed at the input (41). CONTROL UNIT (43) responds to each CHANNEL ADDRESS by incrementing the corresponding channel in a SPECTRUM MEMORY (45). The SPECTRUM-MEMORY channel organization is represented by memory locations that increment every time they are addressed by the corresponding CHANNEL ADDRESS. Typically, the CHANNEL ADDRESS is an address of a physical location in the memory.
An ARITHMETIC UNIT (47) is used to increment the contents of the channels. In most cases, the channel content increments by one. In some circumstances, the increment amount may be different from one. U.S. Pat. No. 6,327,549—Bingham et al. discloses a system that has two banks of histogram memory. Both banks are addressed by the same CHANNEL ADDRESS, but the respective increment amounts for each of them are different. The recorded spectra in SPECTRUM MEMORY (45) are read through the output (49) by user interface for visualization, and can be processed by application software for analysis and storage.
The conventional SPECTRUM MEMORY UNIT operates using a technique that is well known and has been published in various textbooks and technical papers
[Knoll, Nicholson and others as cited]. FIG. 3 shows the flowchart of the conventional method of operation of a SPECTRUM MEMORY UNIT using a conventional spectrum-building method. The flowchart has the following variables:
-j =CHANNEL ADDRESS of the SPECTRUM MEMORY;-ch[j] =content of the j channel of the SPECTRUM MEMORY; and-inc =channel increment value, typically equal to one.Once the operation starts (termination 100), the conventional spectrum-building method checks (decision 110) for the presence of a new CHANNEL ADDRESS from the PULSE-HEIGHT ANALYZER. If there is no new, available CHANNEL ADDRESS, the algorithm checks whether data acquisition is enabled (decision 140). If data acquisition is not enabled, “stop state” is entered (termination 150). If data acquisition is enabled, the algorithm returns to check for a new CHANNEL ADDRESS (decision 110). When available, the new CHANNEL ADDRESS j is received by the SPECTRUM MEMORY UNIT (step 120). Once the CHANNEL ADDRESS j is known, a spectrum-memory operation (step 130) ch[j]=ch[j]+inc is performed to increment the channel content corresponding to CHANNEL ADDRESS j. This concludes the processing of a given CHANNEL ADDRESS. If more pulse heights are to be acquired (decision 140), the entire sequence repeats by returning to the point where the presence of a new CHANNEL ADDRESS is checked for (decision 110). Upon data-acquisition termination, the stop state (termination 150) is entered.
The following is a summary of the main steps of the conventional spectrum-building method:                a) Monitor for a new CHANNEL ADDRESS as long as the spectrum acquisition is enabled;        b) If a new CHANNEL ADDRESS is available, receive the CHANNEL ADDRESS;        c) Increment the “channel content” based on the CHANNEL ADDRESS; and        d) Repeat the sequence as long as spectrum acquisition is required.The salient feature of this widely-used method is the fact that the increment of the spectroscopy channels is based on a single pulse-height measurement. That is, every time a CHANNEL ADDRESS is received, the corresponding channel content increments.        
FIG. 4 illustrates the operation of the conventional SPECTRUM MEMORY UNIT. In this particular example, a PULSE is processed by the PULSE-HEIGHT ANALYZER. The result of the pulse-height measurements is a CHANNEL ADDRESS equal to 996. The SPECTRUM MEMORY UNIT increments the channel content of channel 996. Before the pulse-height measurement, the content of channel 996 is 5432. After adding the increment amount (one in this case) the content of channel 996 becomes 5433. This amount is stored in the
SPECTRUM MEMORY. This amount will remain in the memory until another pulse-height measurement generates a CHANNEL ADDRESS equal to 996 or until the SPECTRUM MEMORY is erased by the user.
An example of a radiation spectrum is shown in FIG. 5. The horizontal axis is the CHANNEL ADDRESS. The vertical axis represents the channel content, i.e. the number of counts. The most important characteristic of the spectroscopy system is its resolution (energy, time etc.) The “full width at half maximum (FWHM)” of the spectral peaks is defined as the width of the peak at half (0.5 H) of its maximum (H). FWHM is a measure of the spectroscopy system's capability to resolve incident-radiation spectral characteristics such as energy. The FWHM depends upon various factors: statistical fluctuations of the detector signal, noise contribution of the signal processor, external interference, temperature, and long-term drifts etc. [Knoll].